The Generator Effect (Oxford AQA IGCSE Physics)

Revision Note

The Generator Effect

Inducing a potential difference in a circuit

  • The process of generating a potential difference in a conductor using a magnetic is called electromagnetic induction, or the generator effect

  • There are two methods of inducing a potential difference in a conductor:

    • Moving a conductor within a fixed magnetic field

    • Placing a conductor in a changing magnetic field

  • If the conductor is part of a complete circuit then the induced potential difference produces a current in that circuit

Moving the electrical conductor

  • When a conductor (such as a wire) is moved perpendicular to the direction of the field lines of a magnetic field (which are fixed) the wire cuts through the field lines

    • potential difference is induced in the circuit which creates the current

    • An ammeter detects the current in the circuit

Moving an electrical conductor in a magnetic field

Moving a wire in a magnetic field generates current, GCSE & IGCSE Physics Revision Notes
Moving an electrical conductor in a magnetic field to induce a potential difference and a current

Moving the magnetic field

  • As the magnet moves through a fixed coil, the field lines cut through the turns on the coil

  • This generates a potential difference in the coil and induces a current

Moving the magnetic field relative to the conductor

A magnet moving through a coil induces potential difference in the coil, GCSE & IGCSE physics revision notes
When the magnet enters the coil, the field lines cut through the turns, inducing a potential difference

AC generator & DC dynamo

  • The generator effect can be used to:

    • Generate ac in an alternator

    • Generate dc in a dynamo

  • A simple alternator is a type of generator that uses mechanical work to produce an alternating current

Diagram of an alternator

A rotating coil in a magnetic field produces an alternating current, GCSE & IGCSE Physics Revision Notes
An alternator is a rotating coil in a magnetic field connected to slip rings
  • A rectangular coil is forced (e.g. by rising steam or wind rotating a turbine) to spin in a uniform magnetic field

  • The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings

    • The slip rings and brushes provide a continuous connection between the coil and the ammeter

  • When the coil rotates continuously in the same direction:

    • The coil cuts through the magnetic field lines, so a potential difference (and therefore current) is induced in the coil

  • The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins

    • This is because the induced potential difference in the coil repeatedly changes its direction

    • This continues as long as the coil keeps turning in the same direction

  • The induced potential difference and the current alternate because they repeatedly change direction / polarity

Alternating current produced by alternator

A graph of current against time shows a sinusoidal pattern, with images of the coil's position at different stages, GCSE & IGCSE physics revision notes
A.C. output from an alternator - the current reverses polarity regularly
  • A dynamo is a direct current (d.c.) generator

  • A simple dynamo is the same as an alternator except that the dynamo has a split ring commutator instead of two separate slip rings

Diagram of a dynamo

A dynamo is a coil in a magnetic field with a split ring, GCSE & IGCSE physics revision notes
A dynamo is a rotating coil in a magnetic field connected to a split ring commutator
  • As the coil rotates, it cuts through the field lines

    • This induces a potential difference between the end of the coil

  • The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction

    • This happens each time the coil is perpendicular to the magnetic field lines

  • Therefore, the induced potential difference does not reverse its direction as it does in the alternator

  • Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative)

    • This means the current is always positive (or always negative)

Direct current produced by dynamo

Now the negative part of the sinusoidal pattern is flipped so current is always positive, GCSE & IGCSE physics revision notes
D.C. output from a dynamo - the current is only in the positive region of the graph

Worked Example

A coil of wire is connected to a sensitive voltmeter. When a magnet is pushed into the coil the needle on the voltmeter will deflect to the right as shown in the diagram below.

A magnet is being pushed leftwards within a coil of wire. Both ends of wire forming the coil are connected to a voltmeter. The voltmeter shows a dial with a 0 V reading in the centre and the needle is deflected towards the positive side of the dial.

What will happen to the pointer on the voltmeter when the magnet is stationary in the centre of the coil?

A      The needle will deflect to the left

B      The needle will deflect to the right

C      There will be no deflection of the needle

D      The needle will deflect to the left and then to the right

Answer:  C

  • C is correct because there the magnet is stationary

  • This means there is no relative movement between the coil and the magnetic field, therefore no magnetic field lines are being cut

  • If the magnetic field lines are not being cut then there will not be a potential difference induced

  • AB & D are incorrect because a deflection on the voltmeter would indicate that a potential difference has been induced

  • This could only happen if there was relative movement between the coil and the magnetic field

Examiner Tip

While it is valuable to be aware of the inner workings of the alternator and dynamo, you will not be asked to recall details about slip rings and split ring commutators. Just make sure you have an understanding of how the system works overall.

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